Modulation of the intestinal transport of calcium by a variety of agents including vitamin D and several hormones has been recognized for many years, yet the nature of the control of the transport system as well as the details of the transport mechanism itself remain largely unknown to date. Using both biochemical and biophysical techniques to study calcium transport in the intestine, it is hoped that the proposed research will elucidate the physiological roles of vitamin D and other hormones on calcium transport at the tissue, cellular, and subcellular levels. Insights derived from these studies should contribute as well to the basic understanding of intestinal transport of ions and water as well as principles related to the control of solute movement across epithelial tissues in general. The present investigation is directed toward the continued evaluation of factors such as vitamin D, cyclic nucleotides, and hormones such as parathyroid hormone in the modulation and control of intestinal calcium transport. Parallel pathways for ion flux through the intestinal epithelium will be investigated to distinguish sites of action of control factors on passive permeability properties of cellular and extracellular (paracellular) pathways versus specific effects on cellular mediated transport deriving energy either primarily or secondarily from cellular metabolism. The net secretory fluxes of calcium in both ileum and duodenum will be investigated to determine the significance of these fluxes in net calcium balance. Measurements of calcium absorption and secretion will be carried out with intact tissue as well as the subcellular level using membrane vesicles. both unidirectional influx at the brush border as well as voltage-clamped transepithelial calcium fluxes will be measured. In addition a paracellular probe methodology will be applied to transepithelial studies. Measurements of calcium fluxes in isolated membrane vesicles of both microvillus and basolateral origin will also be continued. Finally, the role of cyclic nucleotide dependent protein kinase in response to vitamin D will be investigated.